The present invention relates to a vibrating inertial angular sensor such as a gyrometer or a gyroscope and a balancing method of such sensor.
The invention more specifically relates to micro-electromechanical, also called MEMS for “micro-electromechanical system” sensors. Such sensors are obtained by collective etching of a plate made of a very thin material: they are small, lightweight and relatively inexpensive, which makes it possible to use these in many fields of application, both for specialized technical products and for convenience products.
The fields of application for such sensors include the inertial measurement of angular quantities such as a position (gyroscopes) and a speed (gyrometers).
The vibrating inertial angular sensors of the MEMS type can be categorized in two families according to the resonator structure. The resonator may be a deformable body, usually of revolution: a ring, a cylinder, a hemisphere, a disk. The resonator may also consist of one or more non-deformable mass bodies (also commonly referred to as masses or test masses) connected to a substrate by elastic elements. The substrate is rigidly fixed in a rack as disclosed in US-A-2011/0094302 and US-A 2004/0123661. Each sensor comprises actuators so arranged as to vibrate the deformable resonator or the mass body/elastic elements system at the system resonance frequency and detectors detecting the deformations of the deformable resonator or the movements of the mass body/elastic elements system are mounted between the substrate and the deformable resonator on the one hand or the body mass/elastic elements system on the other hand.
The performances of any vibrating inertial sensor directly result from the stability of the damping anisotropy of the resonator. This stability is conditioned by:                the time constant τ of the resonator (equal to the mechanical surge Q divided by π and by the frequency f i.e. τ=Q/(π·f), which the amount of energy that will be required to maintain the resonator in resonance depends on        the dynamic balancing of the resonator, on the one hand, to reduce the energy losses outside of the sensor and, on the other hand, to minimize the disruptions in the vibration of the resonator caused by the vibrational environment of the sensor at the working frequency.        
In the field of MEMS vibrating sensors consisting of one or more non-deformable mass bodies, this results in:                using silicon as a material to obtain a relatively high surge,        the presence of at least two mass bodies symmetrically mounted so that the mass bodies move in phase opposition thereby providing a first-order balancing.        
The best performing MEMS vibrating angular inertial sensors thus have four mass bodies arranged in a square pattern.
The improved performances of such sensors are however limited by the sensors production defects.
Such performance defects cause a dynamic unbalance resulting from the movement of the overall centre of gravity of the mass bodies at the vibration frequency. Such dynamic unbalance causes reaction forces in the substrate and the rack, and thus a loss of energy in the vibration. This is all the more annoying since the small size of the sensors increases the impact of the production defects on the measurements accuracy. As a matter of fact, as for MEMS, the [production defect/characteristic dimensions] ratio is degraded as compared to a macroscopic sensor. This leads to a higher dynamic unbalance relative to the mass of the resonator.
The low mass of the resonator results in that it makes it difficult to measure dynamic balancing defects because the stresses generated by the unbalance are too small to be measured. Moreover, even though such measurement could be achieved, it would be difficult to correct the unbalance by locally removing material because of the small size of the sensor. Such a correction by removing material would further have the disadvantage that it would not make it possible to compensate the unbalance evolution as a function of temperature and time.
Rigidly fixing an unbalanced resonator on a significant recoil mass at the expense of sensitivity to vibration and mechanical strength is recommended.
In the case of resonators with several mass bodies, independence of the mass bodies is further achieved. The mechanical coupling allowing the first-order compensation of the movements of the mass bodies is then provided by levers connecting the mass bodies together to impart these a phase opposition motion. Producing the sensor is then complex and costly. Balancing is also made difficult by the increasing number of degrees of freedom resulting from the number of mass bodies and the number of coupling levers between the mass bodies that cause the influence on the other mass bodies of any balance correction performed on one of the mass bodies. For the same reasons, an electronic balancing by means of a correction algorithm is complex to achieve.